The production of glass-ceramic articles containing synthetic micas as the predominant crystal phase is well known to the art. Hence, whereas naturally occurring micas are typically hydroxyl silicates, micas formed synthetically have customarily involved replacing the hydroxyl group within the crystal lattice with fluorine. Those crystals, often termed fluormicas, have been developed in glass-ceramic articles and, although such fine-grained, polycrystalline articles do not exhibit the single crystal capability of flexibility, they can, however, demonstrate excellent dielectric properties, thermal stability, and mechanical machinability.
In general, the structure of fluormica has been deemed to be defined by the postulated structural formula X.sub.0.5-1 Y.sub.2-3 Z.sub.4 O.sub.10 F.sub.2, wherein X designates cations of relatively large size, i.e., having an ionic radius of about 1.0-1.6 A, Y represents somewhat smaller cations, i.e., having an ionic radius of about 0.6-0.9 A, and Z signifies small cations, i.e., having an ionic radius of about 0.3-0.5 A, which coordinate to four oxygens. In general, the X cations will normally be potassium, but other large alkali metal ions such as Na.sup.+, Rb.sup.+, and Cs.sup.+ and, more rarely, alkaline earth metal ions such as Ca.sup.+2, Sr.sup.+2, and Ba.sup.+2 may be substituted in whole or in part for the potassium ions. The Y cations will commonly be selected from the group of Mg.sup.+2, Li.sup.+, and Al.sup.+3 ions, and the Z cations will be selected from the group of Si.sup.+4, Al.sup.+3, and B.sup.+3 .
The development of glass-ceramic bodies capable of being shaped utilizing hand and machine tools was disclosed in U.S. Pat. No. 3,689,293. Those mechanically machinable glass-ceramic bodies contained synthetic fluormica crystals and consisted essentially, expressed in terms of weight percent on the oxide basis, of about 25-60% SiO.sub.2, 15-35% R.sub.2 O.sub.3, wherein R.sub.2 O.sub.3 consists of 3-15% B.sub.2 O.sub.3 and 5-25% Al.sub.2 O.sub.3, 2-20% R.sub.2 O, wherein R.sub.2 O consists of 0-15% Na.sub.2 O, 0-15% K.sub.2 O, 0-15% Rb.sub.2 O, and 0-20% Cs.sub.2 O, 4-20% F, and 6-25% MgO+Li.sub.2 O, consisting of 4-25% MgO+0-7% Li.sub.2 O.
X-ray diffraction analyses of those products indicated that the basic mica structure consisted of a fluorophlogopite solid solution, this solid solution being posited to be encompassed within three components, viz., normal fluorophlogopite, KMg.sub.3 AlSi.sub.3 O.sub.10 F.sub.2, boron fluorophlogopite, KMg.sub.3 BSi.sub.3 O.sub.10 F.sub.2, and a subpotassic aluminous phlogopite thought to approximate K.sub.0.5 Mg.sub.2 Al.sub.0.83 BSi.sub.3 O.sub.10 F.sub.2.
U.S. Pat. No. 3,756,838 describes the preparation of glass-ceramic articles wherein the predominant crystal glass is an alkali metal-free fluormica. The articles consist essentially, expressed in terms of weight percent on the oxide basis, of about 30-65% SiO.sub.2, 5-26% Al.sub.2 O.sub.3, 10-35% MgO, 3-30% RO, wherein RO consists of 3-30% SrO and 0-25% BaO, and 3-15% F. The patent notes that up to several percent individually of a number of metal oxides may optionally be included, but the total of all such additions will not exceed 10% by weight. Those additions were selected from the group of As.sub.2 O.sub.3, B.sub.2 O.sub.3, BeO, CaO, Fe.sub.2 O.sub.3, La.sub.2 O.sub.3, MnO, PbO, P.sub.2 O.sub.5, Sb.sub.2 O.sub.3, SnO.sub.2, TiO.sub.2, ZnO, and ZrO.sub.2. K.sub.2 O, Rb.sub.2 O and Cs.sub.2 O will be avoided because of their ready substitution for BaO and SrO.
The products are mechanically machinable and contain fluormica solid solutions varying over the range of RMg.sub.2.5 AlSi.sub.3 O.sub.10 F.sub.2 and R.sub.0.5 MgAlSi.sub.3 O.sub.10 F.sub.2. It was observed that the presence of Sr.sup.+2 ions was necessary in the initial batch to stabilize precursor glass formations. Thus, the complete substitution of Ba.sup.+2 ions for Sr.sup.+2 ions causes the melt to quickly and spontaneously devitrify as it is being cooled. Where less than 5% BaO is included in the compositions, the glass-ceramic bodies swell when contacted with water, leading to subsequent disintegration. Moreover, Sr.sup.+2 -containing fluormicas and their intermediates with Ba.sup.+2 additions are quite prone to develop cracks as the precursor glass body is crystallized in situ during heat treatment thereof. Thus, the articles almost invariably develop concentric cracks, the origin of which is not well understood. A study of the crystallization process at progressive stages has augured the hypothesis that a crystallizing front advances from the side of the glass body toward the center, causing the concentric cracks to appear due to density differences between the mica crystals and the residual glass.
Yet, because of the absence of alkali metals from the compositions, the electrical properties of the alkaline earth metal fluormica glass-ceramics are far superior to those glass-ceramics containing "conventional" fluorophlogopite crystals. Furthermore, the mechanical strengths of the alkaline earth metal fluormica glass-ceramics appear to be consistently greater than those demonstrated by those glass-ceramics containing alkali metal fluormicas.